The present invention relates to a magnetron sputtering apparatus, and, more particularly, to an improved planar magnetron sputtering cathode apparatus.
Magnetron sputtering is well known and widely used in production and research applications for the deposition of thin films of various metallic, semiconductor, and ceramic materials on a substrate. A planar target cathode is mounted in a vacuum chamber, and is eroded on one of its surfaces by a DC or RF/AC plasma discharge confined in close proximity to its surface by a closed loop magnetic tunnel. The target, generally a circular or rectangular plate fabricated of the material to be deposited, is electrically connected to the negative side of a DC or biased RF power supply. The positive side of the power supply is connected to a separate anode structure, or may be connected to the vacuum chamber itself if the chamber is electrically conductive. A substrate work piece, the object on which a thin film coating will be deposited, is placed in close proximity to the target cathode. The vacuum chamber is evacuated and a sputtering gas is introduced at low pressure, generally in the range of 10xe2x88x922 to 10xe2x88x924 Torr, to provide a medium in which a glow discharge plasma can be initiated and maintained. The most common sputtering gas is argon (Ar). In some cases, gases or gas mixtures other than Ar are introduced to the vacuum chamber. For example, in a reactive sputtering process, a deposition compound is synthesized by sputtering a selected target material in the presence of a reactive gas. Deposition of a thin film of TIO2 on a substrate work piece by sputtering metallic Titanium in the presence of an Ar/O2 plasma is one common reactive gas sputtering process. When an electric field of direct voltage is produced by the power supply, the electrons generated move under the influence of the electric field, ionizing the introduced gas molecules and thereby producing within the chamber a plasma of positive gas ions, secondary electrons and ions, desorbed gases and photons. The positive gas ions within the plasma are attracted to and impact the target cathode, causing mainly neutral target atoms and secondary electrons to be ejected from the target surface by kinetic energy transfer. The dislodged neutral target atoms impact and condense on the substrate, forming a thin film of the target material or its reactants.
It was recognized early in the development of thin film sputtering processes that utilizing only an electric field (diode sputtering), while producing uniform thin films, resulted in a low rate of deposition. The total number of ions bombarding the target surface during a given time period was relatively low, yielding a low sputtering rate and, consequently, a low rate of deposition. Therefore, this method was not suitable for rapid thin film deposition or to form relatively thick deposition layers.
To improve the deposition rate, the so called magnetron sputtering method has been introduced, wherein a magnetic field is superimposed on the electric field within the sputtering chamber. The magnetic field is created by a magnet assembly comprising permanent and/or electromagnets, usually placed behind and in close proximity to the back surface of the target cathode. The magnets are generally oriented such that their magnetic axes are parallel or perpendicular to the plane of the target. The conventional magnet assembly of the prior art comprises a central core and outer magnets, in opposite magnetic orientation perpendicular to a magnetically permeable, planar base pole piece supporting and physically connecting both magnets. The magnetic flux, exiting from one pole and returning to the opposite pole, crosses the target twice, forming the convexly arched magnetic field. When properly placed and oriented, the magnets produce a closed loop tunnel within and immediately above the sputtering surface of the target. Secondary electrons, ejected from the negatively charged target surface by impacting positive gas ions, are trapped in the closed loop magnetic field tunnel of the prior art. Primarily through collisions between these trapped secondary electrons and chamber sputtering gas molecules, positive ionization is increased. The plasma discharge density and the ionization efficiency of the discharge current produced by the electric field are thereby enhanced. The enhanced plasma density increases the sputtering rate of the target material, since the total number of ions available near the target for impact with its sputtering surface has increased. A correspondingly high rate of deposition is achieved.
As previously described above, in order to enhance sputtering efficiency, it is desirable to produce and confine the ions and electrons in the glow discharge plasma as close as possible to the surface of the target material. It is also, however, desirable that the plasma density in the discharge be uniform over as much of the target surface as possible in order to erode the largest possible fraction of the target volume. Sputtering targets are generally expensive to produce. Although spent targets may be reworked into new targets, any increase in target utilization results in direct savings of target cost, and indirect savings of reduced chamber downtime for target replacement. A significant limitation of the above described magnetron prior art is that erosion of the planar target takes place in a relatively narrow band within the tunnel width and along the closed loop shape of the magnetic field.
Secondary electrons, ejected from the target under the influence of the electric field normal to the target surface plane, are ejected nearly perpendicularly. As is well known by those of ordinary skill in the art, the component of the convexly arched magnetic field extending parallel to the target surface deflects the electron movement along the path of the magnetic tunnel, causing electrons within the glow discharge to gain a net velocity, with the magnitude and direction of the electron velocity vector being given by the vector cross product of the electric field vector E and the magnetic field vector B (known as the Exc3x97B drift velocity). In the region just above both poles of the magnet assembly, the arched magnetic field is almost perpendicular to the target surface, resulting in a very small parallel component. Therefore the electrons can easily escape from the magnetic tunnel. As a result, the ionization region is limited to a narrow band across the arched magnetic tunnel width and along its closed-loop path.
Within the tunnel, the interaction of the drift velocity with the component of the magnetic field perpendicular to the target surface causes another force on the electrons, (Vxc3x97B), in the direction perpendicular to both the magnetic flux lines and the velocity. These lateral forces xe2x80x9cpinchxe2x80x9d the electrons in the glow discharge toward the center of the arched tunnel from both sides. This pinching causes the plasma density and, therefore, the sputtering erosion of the target to be highest along the center of the closed-loop path of the magnetic tunnel. As the sputtering erosion proceeds into the target volume, the convexly arched magnetic field, and in particular its perpendicular component, becomes increasingly stronger, causing stronger pinching and typically producing an acute V-shaped erosion groove in the target, centered along the width of the closed-loop path. The fraction of the target volume which has been sputtered by the time the bottom of the erosion groove reaches the back of the target, called the target utilization, is rather low (typically around 10 to 20%) for a conventional magnetron sputtering cathode apparatus.
Various devices in the prior art have been introduced to increase target utilization in magnetron sputtering. For example, electromagnetic sources have been employed to reshape the curvature of the plasma confining magnetic tunnel into a less convex shape, thereby reducing the field component perpendicular to the target surface and increasing the parallel field component responsible for confining the secondary electrons. Electromagnets have also been used by those skilled in the art to apply an oscillating bias to the static field generated by the permanent magnet assembly, thereby varying the point of maximum erosion off center of the confining tunnel and creating an erosion groove less acutely V shaped.
U.S. Pat. No. 4,892,633 to Welty alternatively uses a permanent magnet shunt below the convention magnet assembly to pull down the confining magnetic flux, shaping it into one of slightly concave curvature adjacent the target surface, thereby decreasing the component of the magnetic field perpendicular to the target surface and the resultant (Vxc3x97B) pinching effect. Apparatus and methods have also been introduced which mechanically rotate, oscillate, or periodically index either the permanent magnet assembly or the target itself to expose different areas of the sputtering surface to the point of maximum erosion. Combinations of the forgoing methods have been introduced and practiced by those skilled in the art to increase target utilization and to provide high, uniform rates of deposition.
Significant disadvantages are associated with all of these approaches, however, with the most obvious being the increased complexity and expenses imposed by the additional electrical and/or structural elements included in these sputtering cathode devices. The introduction of additional elements within the sputtering chamber itself may also necessitate shielding, grounding, or isolating these new elements to avoid contamination of the sputtering process. The techniques described above also generally reduce the average magnetic field strength immediately adjacent and within the target, decreasing the ionization efficiency of the discharge current, increasing the power required to sustain the glow discharge field, and reducing the maximum allowable target thickness or, more accurately, the target width to thickness ratio (aspect ratio). Since the central core magnet mounted on the base pole piece and the outer permanent magnets used to generate the static magnetic field are generally the strongest currently available, e.g., Neodymium Iron Boron (NdFeB) or rare earth magnets, such as Samarium Cobalt (SmCo), or ceramic magnets such as Barium or Strontium Ferrite, the loss of average magnetic field strength cannot be compensated for by the use of stronger permanent magnets.
U.S. Pat. No. 5,262,028 to Manley discloses a magnetron permanent magnet structure that attempts to avoid the magnetic field strength reducing effects and the complex mechanical apparatus of the previous attempts to improve magnetron target utilization. Manley""s basic embodiment includes a conventional circular magnet assembly of the prior art including a cylindrical inner central core magnet situated on and concentrically with a circular plate shaped magnetically permeable base pole piece. A plurality of outer magnets are arranged in a ring shaped pattern around the central core magnet. The core magnet and the plurality of outer magnets are magnetically oriented perpendicular to the base pole piece, with their north-south poles extending vertically with respect to the base pole piece, but in opposite magnetic north-south orientation with respect to one another. To this conventional magnet assembly is added a plurality of primary inner magnets arranged in a ring shaped pattern mounted to or slightly spaced from the central core magnet. Each primary inner magnet has its north-south magnetic orientation parallel to the base pole piece, that is, in a horizontal orientation with respect to the base pole piece, and perpendicular to the vertical magnetic orientations of the central magnet and the outer magnets. The Manley assembly produces a magnetic field having magnetic flux lines that form four separate closed-loop plasma-confining magnetic tunnels or lobes (upper, lower, inner and outer). The lower, inner, and outer magnetic lobes confine the secondary electrons in the glow discharge adjacent the target erosion surface, serving the function of the prior art single closed-loop magnetic tunnel. However, the device of Manley is not stable, since the primary inner magnets, which are magnetically aligned horizontally with respect to the base pole piece, tend to flip toward a vertical orientation. Thus, the primary inner magnets must be secured in place, e.g., by gluing, increasing manufacturing costs and complexity. The horizontal orientation of the primary inner magnets also reduces the space available for the cooling water that is necessary in the magnetron assembly to produce higher power and improved production rates. Additionally, in the device of Manley, gaps, or grooves, must be formed in the base pole piece in order to properly form the four magnetic lobes and avoid magnetic shunting through the base pole piece. Forming the base pole piece with the required gaps also increases manufacturing complexity and costs.
Preferred embodiments of the present invention provide a structure that generates a magnetic field having magnetic flux lines which form three separate closed-loop plasma-confining magnetic tunnels or lobes (upper, inner and outer), all located substantially within the sputtering region over a sputtering target surface. The present invention can improve utilization of the sputtered target, and can also deposit the sputtering material, or its reactants, more uniformly on a substrate work piece. Preferred embodiments of the present invention also can utilize thicker target materials with reduced width to thickness aspect ratios. Planar magnetron sputtering apparatus in accordance with certain preferred embodiments have an improved magnetic field shape to reduce the plasma pinching effect associated with prior art magnetrons, while maintaining mechanical and electrical simplicity of the magnet assembly and target. Other objects and advantages of this invention will be apparent from a reading of the following specification and claims taken with the drawings.
Planar magnetron sputtering magnet assemblies according to the present invention include a sputtering magnet assembly positioned adjacent the back surface of a horizontal planar target for generating a magnetic field with magnetic flux lines that define a three lobed sputtering region adjacent the front surface of the target and within the target body. The magnetic flux lines form a disperse, closed-loop, plasma confining field substantially uniform throughout the sputtering region. The magnetic flux lines from three separate magnetic lobes that project into the target region, forming a closed loop tunnel effective in confining the glow discharge within the sputtering region. There is a wide area in the center of the tunnel with little or no perpendicular magnetic field component which could create lateral forces on moving electrons. Therefore, the trapped secondary electrons and positively charged sputtering gas ions circulating in the plasma are confined in a closed loop volume spread out over a wide area, adjacent the erosion surface. The magnetic lobes continue to effectively confine the plasma over a wide area within the sputtering region as the target material is sputtered away.
In accordance with one aspect, a magnetron sputtering assembly includes a pole member that defines a plane and is formed of magnetically permeable material. A center primary magnet is positioned on the pole member and oriented such that a north-south magnetic orientation of the center primary magnet extends substantially perpendicular to the plane of the pole member. An outer primary magnet is positioned on the pole member proximate an outer edge of the pole member, with a north-south magnetic orientation of the outer primary magnet extending substantially perpendicular to the plane of the pole member and opposite to the north-south orientation of the center primary magnet. A center secondary magnet is positioned on the pole member between the center primary magnet and the outer primary magnet, with a north-south magnetic orientation of the center secondary magnet extending substantially perpendicular to the plane of the pole member and opposite to the north-south orientation of the center primary magnet.
In accordance with another aspect, a magnetron sputtering assembly includes a pole member that defines a plane and is formed of magnetically permeable material. A center primary magnet is concentrically positioned on the pole member and oriented such that a north-south magnetic orientation of the center primary magnet extends substantially perpendicular to the plane of the pole member. A plurality of outer primary magnets are positioned on the pole member proximate an outer edge of the pole member, with a north-south magnetic orientation of each outer primary magnet extending substantially perpendicular to the plane of the pole member and opposite to the north-south orientation of the center primary magnet. A plurality of center secondary magnets are positioned on the pole member between the center primary magnet and the outer primary magnets, with a north-south magnetic orientation of each center secondary magnet extending substantially perpendicular to the plane of the pole member and opposite to the north-south orientation of the center primary magnet.
In accordance with yet another aspect, a magnetron sputtering assembly for sputtering a rectangular target includes an elongated pole member formed of magnetically permeable material that defines a plane. An elongated center magnet assembly is positioned on the pole member and includes a pair of first center primary magnets and a plurality of second center primary magnets oriented in linear fashion between the first center primary magnets, with each center primary having a north-south magnetic orientation extending substantially perpendicular to the plane of the pole member. A plurality of outer primary magnets are positioned on the pole member, with each outer primary magnet being substantially equidistant from the center magnet assembly. A north-south magnetic orientation of each outer primary magnet extends substantially perpendicular to the plane of the pole member and opposite to the north-south orientation of the center primary magnets. A plurality of center secondary magnets are positioned on the pole member between the center primary magnet assembly and the outer primary magnets, with a north-south magnetic orientation of each center secondary magnet extending substantially perpendicular to the plane of the pole member and opposite to the north-south orientation of the center primary magnets.
Additional objects, advantages, and novel features of the invention shall be set forth in part in the description that follows, and in part will become apparent to those skilled in the art upon examination of the following, or may be learned by the practice of the invention. The objects and the advantages of the invention may be realized and attained by means of the instrumentalities and in combinations particularly pointed out in the appended claims.